Conventionally, positron emission computed tomography (PET) apparatuses are known as nuclear medicine imaging apparatuses that can make a functional diagnosis in body tissues of a subject.
Specifically, in a PET examination, a compound labeled with a positron-emitting radionuclide is administered to the subject. Such a PET apparatus coincidentally counts a pair of gamma rays (pair annihilation gamma rays) of 511 keV emitted in approximately opposite directions when a positron emitted from the labeled compound binds to an electron and annihilates by using a photon counting detector arranged around the subject. The PET apparatus then performs calculation processing on data of the gamma rays coincidentally counted (coincidence counting information) to reconstruct a PET image.
In recent years, radiation therapy for damaging a tumor has been performed by using a radiation irradiation device that emits a heavy particle beam as a therapeutic radiation. The heavy particle beam is a radiation generated by accelerating particles, such as a carbon ion, a neon ion, a silicon ion, and an argon ion, at high speed. The depth of penetration of the heavy particle beam into a human body is determined by energy supplied by an accelerator. Furthermore, the heavy particle beam releases energy inside the human body by colliding with an electron and an atomic nucleus in the path of travelling. In particular, the heavy particle beam releases energy drastically near the end of the path of travelling (Bragg peak), and stops. In other words, in the radiation therapy using the heavy particle beam, the Bragg peak is used to adjust the energy of the particles by the accelerator such that the particles stop at the portion of the tumor. As a result, it is possible to kill tumor cells alone while having a little influence on normal cells in the path of traveling.
In the radiation therapy using the heavy particle beam, a range (tumor site) determined by a treatment plan using various types of medical images, such as a PET image, an X-ray CT image, and an MRI image, is irradiated with the heavy particle beam. For example, the radiation irradiation device irradiates the range determined by the treatment plan with the heavy particle beam in multi-directions. Furthermore, also known as the radiation therapy using the heavy particle beam is intensity modulated radiation therapy (IMRT) in which, by performing irradiation of the heavy particle beam in any direction while modulating the magnitude of the heavy particle beam, the influence on the normal cells is reduced and the dose is concentrated within the outline of the tumor.
If the energy is equal to or higher than “511×2 keV”, the heavy particle beam generates an electron and a positron by pair production in the path of travelling. The positron thus generated by pair production binds to an electron therearound and annihilates, whereby pair annihilation gamma rays are emitted. In other words, reconstructing a PET image by the PET apparatus in approximately real-time while performing the irradiation of the heavy particle beam by the radiation irradiation device allows a doctor to monitor whether the site determined by the treatment plan is irradiated with the heavy particle beam.
Therefore, PET apparatuses for radiation therapy monitoring have been developed in recent years. For example, a PET apparatus having two flat detectors arranged at positions facing each other with the subject interposed therebetween is known as such a PET apparatus for radiation therapy monitoring.
However, the conventional PET apparatus for radiation therapy monitoring described above fails to perform coincidence counting in all directions of a circumference about the subject, thereby deteriorating the image quality of PET images. Furthermore, in the conventional PET apparatus for radiation therapy monitoring described above, the irradiation angle of the heavy particle beam emitted from the radiation irradiation device is restricted.